As a battery operated industrial truck equipped with a load handling hydraulic device having an electric motor for driving a hydraulic motor, a forklift truck is known for example, in which a hydraulic pump is allowed to function as a hydraulic motor driven by working oil returning from a lift cylinder when a load is lowered and an electric motor connected to the pump is allowed to function as an electric generator to charge the battery to recover the potential energy of the load.
There has been disclosed energy recovering systems, for example in Japanese Laid-Open Patent Application No. 2-169499 and in U.S. Pat. No. 5,505,043, in which the hydraulic pump is rotated reversely by the working fluid pushed out from the lift cylinder in the lowering operation to function as an electric generator.
The system disclosed in said JP 2-169499 is composed such that an electromagnetic clutch is installed between the electric motor and the hydraulic pump, and power transmission between the hydraulic pump and the motor is shut off by the electromagnetic clutch when energy recovering is not possible when the load is lowered.
The hydraulic lift device disclosed in said U.S. Pat. No. 5,505,043 comprises two pairs of a hydraulic pump and a DC motor respectively for supplying working oil to the lift cylinder and to auxiliary means. With the energy recovering system, potential energy of the load can be effectively utilized, however, there is a problem that supplying pressurized oil to other actuators to actuate other devices is not possible by using the same hydraulic pump connected to the electric motor while operation of supplying working oil to the lift cylinder or recovering potential energy of the load is performed.
Further, the lift device disclosed in said U.S. Pat. No. 5,505,043 has disadvantages that a plurality of hydraulic pumps and a plurality of electric motors are needed, construction becomes complicated, and weight is increased.